Fix start/end margins for API 15

briar-android/src/main/res/layout/activity_dev_report.xml

briar-android/src/main/res/layout/author_view.xml

briar-android/src/main/res/layout/checkbox.xml

briar-android/src/main/res/layout/fragment_error.xml

briar-android/src/main/res/layout/introduction_message.xml

briar-android/src/main/res/layout/list_item_blog_post.xml

briar-android/src/main/res/layout/list_item_contact.xml

...r-android/src/main/res/layout/list_item_contact_small.xml

briar-android/src/main/res/layout/list_item_forum.xml

briar-android/src/main/res/layout/list_item_group.xml

briar-android/src/main/res/layout/list_item_invitations.xml

...roid/src/main/res/layout/list_item_revealable_contact.xml

briar-android/src/main/res/layout/list_item_rss_feed.xml

...roid/src/main/res/layout/list_item_selectable_contact.xml

briar-android/src/main/res/layout/list_item_thread.xml

briar-android/src/main/res/layout/navigation_header.xml

briar-android/src/main/res/layout/power_view.xml

briar-android/src/main/res/layout/text_avatar_view.xml

briar-android/src/main/res/layout/text_input_view_large.xml