Only build examples with stable toolchain

Although we want to ensure android-activity itself builds with the
same MSRV as Winit there's no need to also check that the examples
build with an older toolchain, and in fact in the case of Egui based
examples they don't support being built with 1.60.0.

In addition to adding an `if: rust_version == "stable"` condition
for building agdk-egui, this also adds CI stages to build agdk-mainloop,
na-mainloop and agdk-eframe to cover at least one example based on
`NativeActivity` and cover the key examples that others are most
likely to be interested in.

.github/workflows/ci.yml

@@ -67,7 +67,30 @@ jobs:
           -t x86
           build --features native-activity
 
+      - name: Build agdk-mainloop example
+        if: matrix.rust_version == stable
+        working-directory: examples/agdk-mainloop
+        run: >
+          cargo ndk
+          -t arm64-v8a
+          -t armeabi-v7a
+          -t x86_64
+          -t x86
+          -o app/src/main/jniLibs/ -- build
+
+      - name: Build na-mainloop example
+        if: matrix.rust_version == stable
+        working-directory: examples/na-mainloop
+        run: >
+          cargo ndk
+          -t arm64-v8a
+          -t armeabi-v7a
+          -t x86_64
+          -t x86
+          -o app/src/main/jniLibs/ -- build
+
       - name: Build agdk-egui example
+        if: matrix.rust_version == stable
         working-directory: examples/agdk-egui
         run: >
           cargo ndk
@@ -77,6 +100,17 @@ jobs:
           -t x86
           -o app/src/main/jniLibs/ -- build
 
+      - name: Build agdk-eframe example
+        if: matrix.rust_version == stable
+        working-directory: examples/agdk-eframe
+        run: >
+          cargo ndk
+          -t arm64-v8a
+          -t armeabi-v7a
+          -t x86_64
+          -t x86
+          -o app/src/main/jniLibs/ -- build
+
       - name: Documentation
         run: >
           cargo ndk -t arm64-v8a doc --no-deps