Fixed: BackAndroid is deprecated since RN0.44.0^.

[patlux]

RN 0.44.0^: fixed warning 'BackAndroid is deprecated. Please use BackHandler instead'.

Renamed option 'detectBackAndroid' to 'detectBackPress'.
Updated README.md

[fabriziomoscon]

This change work for me on RN 0.46.
@jaysoo at the moment there are 3 PRs for this issue: #47 #48 #61 - showing that people really needs this change.
I think this PR is the most complete for merging.
It represents a breaking change in the library semantic version though, and users or RN < 0.44 must keep using the old version.
I can see the last commit to master happened in April, but it will be very useful to further developing this library that this PR is merged.
For your convenience I want to list the steps that I think should happen:

• merge this PR into master
• bump up a major library version
• add a paragraph in the README explaining the breaking change and what people using RN < 0.44 should do
• publish on npm
• let all the authors of outstanding PRs to rebase in top of master

Thank you so much in advance for your work and maintaining this project.

[jaysoo]

Merged #53. Closing this one.

Thanks!