Add checks to HW codecs to ensure unsupported features are not used.

Add checks to ensure encoder is not used below API level 19. Removes
global @TargetApi from MediaCodecUtils since it is also used by the
decoder. Ensures that texture mode is never enabled below API level 18.

Bug: webrtc:9821
Change-Id: I2ca1014bf8995719c970eb1449b0acbf7b3c883e
Reviewed-on: https://webrtc-review.googlesource.com/c/103701
Reviewed-by: Paulina Hensman <phensman@webrtc.org>
Commit-Queue: Sami Kalliomäki <sakal@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#24990}

sdk/android/src/java/org/webrtc/HardwareVideoEncoder.java

@@ -28,7 +28,11 @@ import java.util.concurrent.TimeUnit;
 import javax.annotation.Nullable;
 import org.webrtc.ThreadUtils.ThreadChecker;
 
-/** Android hardware video encoder. */
+/**
+ * Android hardware video encoder.
+ *
+ * @note This class is only supported on Android Kitkat and above.
+ */
 @TargetApi(19)
 @SuppressWarnings("deprecation") // Cannot support API level 19 without using deprecated methods.
 class HardwareVideoEncoder implements VideoEncoder {