Fix up ModifierShortcutsTests to use test bookmarks.xml

Rather than relying on the device version of bookmarks.xml which
may not align with the shortcuts to be tested in the test case, use
the frozen copy of bookmarks.xml in the test APK.

Flag: TEST_ONLY
Bug: 312452252
Test: atest ModifierShortcutTests
Change-Id: Ica2ff886f3a930cfb75ca5b4889a69eb30f29a7f