Allow adding target activity via requestPinShortcut().

Also now ShortcutManager tries to fill in the main activity as the target
activity if the caller has one.

Test: adb shell am instrument -e class com.android.server.pm.ShortcutManagerTest1 -w com.android.frameworks.servicestests
Test: adb shell am instrument -e class com.android.server.pm.ShortcutManagerTest2 -w com.android.frameworks.servicestests
Test: adb shell am instrument -e class com.android.server.pm.ShortcutManagerTest3 -w com.android.frameworks.servicestests
Test: adb shell am instrument -e class com.android.server.pm.ShortcutManagerTest4 -w com.android.frameworks.servicestests
Test: adb shell am instrument -e class com.android.server.pm.ShortcutManagerTest5 -w com.android.frameworks.servicestests
Test: adb shell am instrument -e class com.android.server.pm.ShortcutManagerTest6 -w com.android.frameworks.servicestests
Test: adb shell am instrument -e class com.android.server.pm.ShortcutManagerTest7 -w com.android.frameworks.servicestests
Test: adb shell am instrument -e class com.android.server.pm.ShortcutManagerTest8 -w com.android.frameworks.servicestests
Test: adb shell am instrument -e class com.android.server.pm.ShortcutManagerTest9 -w com.android.frameworks.servicestests
Test: Running: cts-tradefed run cts --skip-device-info --skip-preconditions --skip-system-status-check com.android.compatibility.common.tradefed.targetprep.NetworkConnectivityChecker -a armeabi-v7a -m CtsShortcutManagerTestCases
Test: cts-tradefed run cts --skip-device-info --skip-preconditions --skip-system-status-check com.android.compatibility.common.tradefed.targetprep.NetworkConnectivityChecker -a armeabi-v7a -m CtsShortcutHostTestCases

Bug 34047091

Change-Id: I2bf2b20e94a2384cfa6e6b7d40d56cfe96a94fd9